The present invention relates to a turbo-type fluid machine provided with a mouth ring that seals a narrow rotation clearance between a non-rotating body such as a pump casing, etc. and an impeller shroud, and a stepped seal apparatus to be used for the mouth ring in the turbo-type fluid machine.
Turbo-type fluid machines comprise a rotating shaft and an impeller mounted to the rotating shaft, and work is done by that difference in angular momentum between an inlet and an outlet of the impeller, which is generated by the impeller being rotated by the rotating shaft. Therefore, except the case of a full open impeller, a narrow rotation clearance or clearances are formed in one or two locations between a non-rotating body and an impeller. A loss is caused by that leakage flow, which flows from a high-pressure side to a low-pressure side in the impeller through the narrow clearance or clearances. Such leakage loss amounts in some cases to 20% as large as a total loss in the case where a fluid machine of high efficiency runs at a low specific speed.
The leakage flow q is in proportion to a cross sectional area A of a narrow clearance and the square root of a differential pressure ΔP between the front and the rear of the narrow clearance and in inverse proportion to the square root of flow resistance ζ in the narrow clearance. Therefore, various considerations are taken to increase the flow resistance in the narrow clearance.q=αA√{square root over (ΔP/ζ)}  (1)where α is a constant.
The flow resistance ζ in the narrow clearance is classified into two kinds, that is, frictional loss generated at the time of flowing through the narrow clearance, and suddenly enlarged loss and suddenly contracted loss, which are attendant on changes of the narrow clearance in shape and cross sectional area. Since the frictional loss is determined by dimensions of the narrow clearance, the velocity of flow of a working fluid, and a coefficient of kinematic viscosity, there are limits in making a clearance extremely narrow and in lengthening the clearance to cause an increase in resistance from the necessity of avoiding contact between an impeller and a non-rotating body. Therefore, a seal apparatus for reduction in leakage loss in a mouth ring part is in most cases structured to effectively introduce changes in shape and cross sectional area, and uses, for example, a staircase-shaped labyrinth seal construction described in JP-A-11-343996.
Since a labyrinth seal construction in a conventional mouth ring part makes use of that adiabatic compression effect, which is produced by sudden area enlargement from a projection of a labyrinth to a recess thereof, for flow resistance, however, no sufficient effect for reduction of leakage flow is obtained in the case where the construction is used for a turbo-type fluid machine such as a pump, etc., in which an incompressible fluid is treated, as it is.
In a turbo-type fluid machine such as a pump, etc., in which an incompressible fluid is treated, it is conceivable to provide a labyrinth construction composed of recesses and projections on a side of an impeller shroud as shown in FIG. 6 to make use of an effect of a secondary flow on the incompressible fluid produced by centrifugal forces generated in the labyrinth to increase the flow resistance. However, this construction involves a problem that an amount of leakage is considerably increased when relative axial positions of a rotating body and a stationary body change. Also, with the construction shown in FIG. 6, projections 4b are smaller in axial dimension than recesses 4a in a impeller ring 4 that constitutes a labyrinth part, by which working is made difficult and working with high precision is also demanded.
Therefore, in a turbo-type fluid machine such as a pump, etc., in which an incompressible fluid is treated, it is conceivable to use a stepped seal apparatus of a multi-stage construction, in which a narrow clearance part is larger in axial length than a clearance part as shown in FIG. 7. In case of using such stepped seal apparatus of a multi-stage construction, however, a ratio of leakage loss in a stepped seal part to a total loss amounts to a magnitude that cannot be disregarded, as described above, and becomes a serious problem in a competition of efficiency at 1% level in the present market.